1. Field of the Invention
The present invention relates to a multilayer-type piezoelectric actuator comprising a multilayer piezoelectric unit including a plurality of alternately stacked piezoelectric plates and internal electrodes and a pair of external electrodes arranged on the sides, respectively, of the multilayer piezoelectric unit.
2. Description of the Related Art
A conventional multilayer-type piezoelectric actuator, as shown in FIG. 10, comprises a multilayer piezoelectric unit 92 including a plurality of alternately stacked piezoelectric plates 921 and internal electrodes 922 and a pair of external electrodes 93 arranged on the sides 923, respectively, of the multilayer piezoelectric unit 92 (Japanese Unexamined Patent Publication No. 10-229227).
The multilayer-type piezoelectric actuator 9 is so configured that a voltage is supplied intermittently to the piezoelectric plates 921 from the external electrodes 93 and through the internal electrodes 922 and, by thus extending or contracting the piezoelectric plates 921 in the direction along the thickness, the whole multilayer piezoelectric unit 92 can be displaced by extension or contraction.
The external electrodes 93 of the multilayer-type piezoelectric actuator 9 are formed by printing, drying and baking a conductive material on the side surfaces 923 of the multilayer piezoelectric unit 92. The external electrodes 93, however, are only as thick as about 50 xcexcm and have a low mechanical strength. For this reason, a reinforcing conductive material such as solder is further applied on the printed conductive material to increase the thickness and thus to secure a practically satisfactory strength of the external electrodes 93.
The external electrodes 93, which are in solid form, cannot endure the extension or contraction of the multilayer piezoelectric unit 92, under voltage, and are liable to develop a crack and break. If the external electrodes 93 are disconnected by breakage, a voltage may not to be applied to all the multilayer piezoelectric unit 92.
The xe2x80x9cdisconnection of the external electrodesxe2x80x9d is defined as the state in which the electrical conduction between the external electrodes and any of the Internal electrodes is cut off and a voltage cannot be supplied to the piezoelectric plates through the internal electrodes.
An attempt to apply a reinforcing conductive material such as solder to thicken the external electrodes 93 to prevent the cracking and breakage thereof, on the other hand, undesirably limits the mechanical extension or contraction of the multilayer piezoelectric unit 92. This poses the problem of a reduced amount of the originally intended extension or contraction of the multilayer-type piezoelectric actuator
The present invention has been achieved in view of the problems of the prior art described above, and the object thereof is to provide a multilayer-type piezoelectric actuator in which the multilayer piezoelectric unit is sufficiently extended or contracted without disconnecting the external electrodes.
According to a first aspect of the invention, there is provided a multilayer-type piezoelectric actuator comprising a multilayer piezoelectric unit including a plurality of piezoelectric plates of a piezoelectric material and a plurality of internal electrodes of a conductive material stacked alternately with each other and a pair of external electrodes arranged on the side. surfaces, respectively, of the multilayer piezoelectric unit and electrically connected to the internal electrodes,
wherein the external electrodes are each configured of an electrode base arranged in contact with the corresponding side surface of the multilayer piezoelectric unit, a tabular metal net conductor arranged on the electrode base and a plurality of conductive adhesives for partially bonding the electrode base and the tabular metal net conductor and
wherein adjacent ones of the conductive adhesives have a mutually superposed portion along the direction parallel to the piezoelectric plates.
What is most noticeable about this invention is that the external electrodes are each formed of the electrode base, the tabular metal net conductor and a plurality of conductive adhesives.
The tabular metal net conductor is a braid of conductive wires of such a material as gold, silver, copper, nickel or stainless steel and is sufficiently flexible along the direction of extension or contraction, i.e. the direction of stacking of the multilayer piezoelectric unit. The tabular metal net conductor need not be braided with conductive wires, as long as it is continuous along the direction of stacking of the multilayer piezoelectric unit and flexible.
Also, the tabular metal net conductor may have what, is called a two-dimensional meshed structure formed by etching flat metal plates (fourth embodiment).
The electrode base is composed of a material such as baked silver, baked copper, baked platinum or baked nickel. The electrode base is formed by baking silver paste, copper paste, platinum paste or nickel paste, respectively, and is composed of, say, 97% silver and 3% glass frit.
The electrode base may alternatively be formed by plating, CVD, sputtering or vapor deposition of, for example, gold, silver, copper or nickel.
The conductive adhesive is composed of resin silver, resin copper, resin platinum or resin nickel and contains 80% silver and 20% epoxy resin, for example.
Now, the functions and effects of the invention will be explained.
The external electrodes are each so configured that the tabular metal net conductor, sufficiently flexible in the direction of extension or contraction of the multilayer piezoelectric unit, is bonded partially to the electrode base by the conductive adhesives. Even in the case where the multilayer piezoelectric unit is extended or contracted, therefore, the tabular metal net conductor is not broken by being extended or contracted following the extension or contraction of the multilayer piezoelectric unit. Should the electrode base be cracked by the extension or contraction of the multilayer piezoelectric unit, therefore, the tabular metal net conductor and the conductive adhesive can secure the conduction of the external electrodes on the two sides of the cracking. Thus, the external electrodes are not disconnected.
Also, the tabular metal net conductor, which is adapted to extend or contract following the extension or contraction of the multilayer piezoelectric unit, does not restrict the extension or contraction of the multilayer piezoelectric unit. As a result, the multilayer piezoelectric unit extends or contracts sufficiently.
Further, adjacent ones of the conductive adhesives have a mutually superposed portion along the direction parallel to the piezoelectric plates. Therefore, the breakage of the external electrodes can be prevented more positively.
The conductive adhesives arranged in the manner described above are present over the entire area of the external electrodes along the direction parallel to the piezoelectric plates.
Cracking, if it develops in the electrode base, runs in the direction parallel to the piezoelectric plates.
Should two cracks run in the electrode base, therefore, the conductive adhesive always exists on the electrode base between the two cracks. Further, the conductive adhesive is bonded with the tabular metal net conductor. As a result, the electrode base between the two cracks is not electrically isolated, so that the corresponding piezoelectric plate is always supplied with a voltage.
By forming the conductive adhesives as described above, the disconnection of the external electrodes can be prevented even more positively.
As described above, the present invention provides a multilayer-type piezoelectric actuator in which the multilayer piezoelectric unit is sufficiently extended or contracted and the external electrodes are not disconnected.
According to a second aspect of the invention, there is provided a multilayer-type piezoelectric actuator wherein each tabular metal net conductor is formed of conductive wires and at least some of the conductive wires are arranged preferably along the direction of stacking of the multilayer piezoelectric unit (FIGS. 2, 3, 6 and 7(A)-7(B)).
In this case, the electrical resistance of the tabular metal net conductor can be reduced along the direction of stacking of the multilayer piezoelectric unit. Should a crack develop in the electrode base at the time of extension or contraction of the multilayer piezoelectric unit, therefore, the conduction of the external electrodes can be sufficiently secured on the two sides of the crack.
The conductive adhesives are preferably formed at an angle to the thickness of the piezoelectric plates. In this case, the conductive adhesives can be easily formed in such a manner that adjacent ones thereof have a mutually superposed portion along the direction parallel to the piezoelectric plates.